The differentiation of multipotential hemopoietic stem cells to functional B lymphocytes will be studied in mice utilizing the following experimental models: 1) embryonic and neonatal mice, 2) adult, irradiated, stem cell reconstituted mice, 3) in vitro culture of stem cell containing tissue fragments, and 4) genetically anemic mice made chimeric with respect to donor stem cells. Various cell surface markers and functional assays will be employed to detect differentiating B cells and assess their functional maturity. These studies will be conducted with the following specific aims: 1) delineation of tissues essential to the differentiation of the plasma cell line from stem cells in mammals, 2) identification of discrete stages in the development of B lymphocytes and characterization of the various cell types in terms of immunoglobulin synthetic, differentiative, proliferative, and functional potential, and 3) analysis of the duration and kinetics of differentiation of hemopoietic stem cells through discrete maturation stages to functional B lymphocytes.